This invention relates to electronic reference circuitry. More particularly, the invention relates to bandgap references that provide a substantially constant output current which may be used as voltage or current references.
Bandgap voltage references have been widely used in electronic applications for many years. The purpose of a bandgap voltage reference is to provide a substantially constant and stable voltage over a fairly wide temperature range. Such references form a vital part of numerous commonly used circuits such as analog-to-digital and digital-to-analog converters, phase-locked loops, voltage regulators, comparison circuits, etc.
The basic principle behind the bandgap reference is the well-known voltage drop associated with certain semiconductor junctions. For example, a silicon p-n junction such as the emitter-base junction bipolar transistor may have a forward conduction characteristic (i.e., voltage drop) of about 0.6 volts. It is possible to construct a basic voltage reference circuit based on this known physical conduction property. For example, one or more such p-n junctions may be connected in series to form a voltage reference circuit that has a predetermined and stable output voltage. For example, connecting two silicon diodes in series provides a regulated 1.2 volt output, three silicon diodes connected in series provide a regulated 1.8 volt output, etc.
Although the configuration proposed above does provide a stable reference voltage, it is well known that the forward conduction characteristics of semiconductor junctions change with temperature. As temperature rises, the forward voltage drop is altered, resulting in a negative temperature coefficient, which undesirably changes the output voltage. Similarly, as temperature falls, the forward voltage is also altered, resulting in a positive temperature coefficient, which also undesirably alters the output voltage, albeit with an opposite effect.
Improved bandgap voltage references have been proposed which employ various compensation schemes that attempt to normalize output voltage over a wide temperature range. Such bandgap reference circuits are transistor-based and operate on the principle of compensating the negative temperature coefficient of a base-emitter voltage (VBE) of a bipolar transistor with the positive temperature coefficient of the thermal voltage, (i.e., with VThermal=k*(T/q), where k is Boltzmann's constant, T is the absolute temperature in degrees Kelvin, and q is the electronic charge). In general, the negative temperature coefficient of the base-emitter voltage VBE is summed with the positive temperature coefficient of the thermal voltage VThermal, which is appropriately scaled such that the resultant summation provides a small or negligible temperature coefficient over a fairly wide temperature range.
More specifically, a reference voltage is typically obtained by combining two generated voltages having equal and opposite temperature coefficients (TC). One is the base-emitter voltage (VBE) of a forward biased bipolar transistor QREF with a TC of about −2 mV/° C. This voltage is said to be complementary to absolute temperature voltage (VCTAT) and can be expressed as:VCTAT=VBE(TR)−VG0−[(VG0−VBE(T0))*(T/T0)]+[(kT/q)*(n−m)*ln {T/T0)]  (1)where VG0 is the extrapolated bandgap voltage at 0 degrees K, and n and m are process related parameters representing, respectively, the temperature variation of mobility and collector current. T0 is the temperature at which VBE is measured, T is the Kelvin temperature, k is Boltzmann's constant, q is the charge on the electron, and VBE(TR) is the base-emitter voltage at the reference temperature TR.
To generate the bandgap, reference circuits typically employ two groups of transistors running at different current densities. For example, one group of transistors will typically run at about ten times the current density of the other group. This causes a 60 mV difference between the base-emitter voltages of the two groups. This difference in voltage is usually amplified by a factor of about ten and is added to the base-emitter voltage. The total of these two voltages typically adds up to about 1.22 volts, which is essentially the bandgap of silicon.
A typical prior art bandgap circuit 100 is shown in FIG. 1. Bandgap circuit 100 generally includes an NPN transistor 160 that runs at a relatively high density. NPN transistor 170 is operated at a lower density, thus the voltage at the emitter of transistor 170 is approximately 60 mV. This voltage is applied across resistor 150 and is increased by the ratio of resistor 140 to resistor 150. If the ratio is approximately ten to one, the voltage level moves up to approximately 600 mV. This voltage is added to the base-emitter voltage of NPN transistor 180, producing a total voltage of about 1.22 volts. Transistor 180 then amplifies the error signal through transistors 125 and 190, which provides enough gain to shunt regulate the output voltage between nodes V+ and V− at 1.22 volts.
Such conventional bandgap circuits however, are typically concerned with providing a substantially constant output voltage. Moreover, output voltage in conventional bandgap circuits is dependent on certain transistor conduction characteristics, current gain (i.e., beta), and therefore subject to change due to process and other variations associated with physical implementation. Moreover, the minimum output voltage of such references is about one bandgap, or 1.22 volts.
Accordingly, in view of the foregoing, it would be desirable to provide improved reference circuitry that overcomes these and other drawbacks.